Energy calibration in semiconductor detectors (also called photon counting detectors, as e.g. known from WO 2014/087290 A1) is conventionally performed by use of gamma ray sources of known energy or by use of K-edge filters.
In the method using gamma ray sources the pixel array is irradiated with a mono-chromatic gamma ray source of known energy. The irradiation needs not be homogenous on the entire pixel array. During the irradiation a so-called threshold scan or threshold sweep is performed. This measurement establishes a relationship between the most likely pulse height observed, the so-called “photo-peak” in the pulse-height spectrum (PHS) (typically measured in DAC (digital-to-analog-conversion) values) and the incoming energy. Gain and offset variations and position dependent incomplete charge collection all lead to measureable and significant variations in the DAC-position of the photo-peak. In order to determine gain and offset on the calibration curve between energies and DACs, a second measurement is required. This second measurement can come from a second monochromatic source with sufficiently distinct energy from the first. Under the assumption of a linear dependence between the measured DAC of the photo-peak and the incoming energy, two energy milestones are sufficient to determine offset and gain for each channel.
The method using K-edge filters is very similar. However, no gamma ray sources are needed to calibrate the energy scale. Instead, a polychromatic source spectrum can be used and the energy milestone in this case is identified with a strong change in the attenuation of a K-edge filter material. Again identified from the threshold scan under polychromatic illumination, the extracted feature allows identifying one energy milestone on the DAC scale. The same options exist for the completion of the energy calibration as in the above method, namely performing a second measurement using a different K-edge filter.
An alternative second measurement for both of the above methods makes use of the existence of a noise peak in the pulse height spectrum. The noise peak results from the counting of the electronics shot noise when the comparator is moved into the noise band at the baseline. It is an assumption that the maximal count rate observed in the absence of any radiation hitting the sensor occurs for a DAC value for which the analog signal DC coincides with the location of the threshold. This alternative is useful in combination with one of the above methods but cannot be used to perform a complete energy-calibration because it provides only one reference energy.
The problems with above two methods of energy calibration are numerous. The use of gamma ray sources is problematic due to the radiation aspect and due to continuously changing activity of the source. Besides that a very close distance needs to be maintained during the calibration measurement which is unpractical for an entire detector array like the ones used for computed tomography or even mammography. Further, this method suffers from low x-ray fluxes and, hence, long calibration times. The use of K-edge filters is more practical because x-ray tubes can be used. However, the extraction of the K-edge attenuation feature is less trivial and often leads to erroneous measurements of the energy reference. Further, polychromatic sources combined with K-edge filtration suffer from the difficult process of extracting the K-edge feature from the measured spectra, in particular for realistic spectral response functions, like observed in millimeter-sized CdTe of CZT detectors, most likely candidates for future photon counting CT scanners.
Hence, it would be desirable to have an alternative measurement option for a second energy reference in addition to the above mentioned noise peak in the pulse-height spectrum (PHS; i.e. the relative frequencies of measured pulse heights) to obtain the measure the gain- and/or offset-variations in those detectors.
DE 10 2010 015422 A1 discloses an x-ray detector including a directly converting semiconductor layer for converting an incident radiation into electrical signals with a band gap energy characteristic of the semiconductor layer, and at least one light source for coupling light into the semiconductor layer, wherein the generated light, for the simulation of incident x-ray quanta, has an energy above the band gap energy of the semiconductor layer. In at least one embodiment, it includes at least one evaluation unit for calculating an evaluation signal from the electrical signals generated when the light is coupled into the semiconductor layer, and at least one calibration unit for calibrating at least one pulse discriminator on the basis of the evaluation signal. This provides the prerequisites for a rapidly repeatable calibration of the x-ray detector taking account of the present polarization state without using x-ray radiation. At least one embodiment of the invention additionally relates to a calibration method for such an x-ray detector.